Process of treating zinc oxides



lApu] 8, 1952 J H CALBECK v PROCESS oF TREATING ZINC oxIDEs Original Filed Feb. 26, 1945 Maali llllllllllllldl llllillllllllllll A es Sii!! Ressued Apr. 8, 1952 `'PROCESS OF TREATING ZINC' OXIDES John Henry Calbeck, Columbus, Ohio, assigner to American Zinc, Lead & Smelting. Company, St. Louis', Mo.. a. corporation. of Maine Original No. 2,416,044, dated" February 18', 1947',

Serial No. 477,305, February 26, 194-3. Application for reissue December 19, 1951-, Serial No.

l2 Claims.

This invention relates to a process for treating zinc oxide; and has for its objects, among others, to improve color. to remove objectionable sulfur compounds, to rene and at the same time not destroy the texture, oil absorption or other physi'caly properties of the oxide, and especially to recover therefrom as a valuable by-product without substantial Volatilization of any metallic zinc the small percentages of cadmium that are present in most commercial zinc oxides and which have heretofore 'been allowed to remain in the oxide, and to secure a final zinc oxide that is chemically pure.

A further object is to secure these results by reheating zinc oxide under conditions of controlled temperatures, time and atmospheres.

Another object is to improve the color of American process or other zinc oxides by reheating the same at relatively low temperatures by a heating name which is maintained clean and free of soot or solid carbon; and to procure a white non-densiiied Zinc oxide-by treatment o'f the zinc oxide in a'high velocity gaseous environment to remove changing the texture of the zinc oxide regardless of whether the originalntexture was light and iiuiy or heavy.

Another object is to provide a continuous method, for treating zinc o-xide in an air-sealed furnace. wherein the flame is alternated from an oxidizing one to a reducing one or vice versa, it being-.immaterial whether the start is with the oxidizing or reducing if the intervals are short, and the reaction zone in the. furnace is of sufficient length. and the movement of the oxide. is a Commercial zinc oxide for use as. a pigment is.-

produced by oxidizing the vaporsv obtained by the volatilizati'on o f zinc. Whenl zinc. metall is used.. an oxide. of high chemical purity is pro..-

impurities Without Matter enclosed in heavy brackets appears in the original patent but forms no part oi.' this reissue specification; matter printed in italics indicates the additions made by reissue.

duced and` is;` known commercially as French process zinc oxide.. Such oxides requireI no, further: treatment. Of greaterimportance commercially is the type oef zinc oxide known as American process which is manufactured from zinc ores. American process zinc oxidev is made by burning a mixture of Zine oreand carbonaceous fuel on a flat grate. The zinc content of the ore is reduced to metallic zinc vapo-rs which escape with the products of combustion, and these vapors are oxidized` with air in a suitable combustion chamber. A, smoke or fume is produced which consists of minute solid` particles of zinc oxide in suspension in a hot gaseous mix'- ture. This. fume is cooled byI passing through pines andis filtered inA .bags to recover the zinc oxide. American process zinc oxide may contain small amounts. of impurities. that have been present in the ore suchas sulfur compounds, cadmium.. lead',..arsenic. andv theV like, depending on thev character of. the ore. fromE which the oxide is obtained. If obtainedv froml very pure ores American process zinc. oxide may be suitable for many purposes and may bez packed andV Sold Without.. iurther treatment.

In order to` produce, an American process zinc oxide, of the highest quality it is necessary to rene or reheat it, regardless, of how pure the ore may be from. which it is. produced. The 4primary purpose of refining heretofore has been to improve the color.` Second i'n. importance hasbeen the removal of objectionable. sulfur compounds from the. oxide which. necessary when ores.

high in. sulfur.. are usedj..

Agreat number of methods for conducting this reheating step. are, described in the patent. literature. In UL S. Patent No. 406,868 (.1889). Bartlett, describes ay reheating process in which he teaches relocatingA in. the, presence of. currents` of air at a temperature not exceeding l0U0 F. (538 C.) with. constant agitation. Later the same inventor, U; S. Patent No. 480,686; modi'- fied his process by excluding` air and substituting a sulfurous atmosphere, the purpose of which Wasto sulfate any lead or cadmium that might be present in the oxide and thereby improve the color of the product. Still later, in 1902, the same inventor, U. S. Patent No. 715,238, taught the use of a reducing atmosphere produced by burning coal with a deciency of air. In this process the zinc oxide or fume was reheated in batches in a reverberatory furnace. The oxide, when rst introduced into the furnace. was exposed to a very smokey flame which was passed over the oxide until it was thoroughly contaminated with a carbonaceous material. Then the heating was continued with a flame free of solid carbon particles until all the carbonaceous material had been burned out. Thus was introduced to the art for the first time the device for adding solid carbonaceous' materia1 to zinc oxide prior to reheating.

The early inventors reheated the zinc oxide v cation which comprised passing the oxide by theaction of gravity in the form of a thin stream through a highly heated atmosphere. No carbonaceous matter was added and the furnace was designed to prevent the admission of air so that the entire treatment was in a stagnant atmosphere. Later, the same inventors. U. S. Patent No. 1,339,545, in a similar type of apparatus, provided an adequate supply of oxygen to the zinc oxide while the latter is undergoing the reheating treatment primarily to improve the color.

In 1919, Wemple, U. S. Patent No. 1,292,976, described a process for reheating zinc oxide wherein the coarsening of grain due to excessive heating was avoided." He taught that if a solid carbonaceous material, such as lamp black, was a'dded to the zinc oxide in proper amount prior to reheating, obiectionable sulfur compounds could be removed, the color would accordingly be improved, and no objectionable densification would occur because of the relatively low temperatures required. Furthermore, Wemple taught that the cadmium and lead contents were necessary to the production of `a good color and he made every effort to retain theseimpurities in the oxide and even advocated the addition of some cadmium or lead to the product to absorb the sulfatos in .case there vwas not suflicient present as a natural impurity. Wemple states Having obtained the crude product as described, to rene the same for use as a pigment, it is necessary only to eliminate the soluble zinc salts [zinc sulfate] and it may be understood that this prpcess of elimination must leave the lead and cadmium present unchanged in the unobjectionable form of sulfates. been successful-and has been used without interruption for reheating all the lead-free zinc oxide produced by the American Zinc, Lead & Smelting Co. since 1919.

n There are many contradictions in the prior art. One inventor teaches the use of an oxidizing atmosphere, another teaches the use of a reducing atmosphere, one teaches that the product must be kept free from contamination from any of the products of combustion of the furnace, While another advocates the use of these products to as# sist in the refining operation. `One proposed to This y process has prevent densication by agitation, another by the use of carbon.

The present invention involves a simple, novel and economical procedure comprising reheating zinc oxide in an atmosphere which is controlled with respect to temperature, composition and velocity, so as to improve the color of the zinc oxide, remove sulfur compounds and other impurities, control and not destroy the oil absorption, texture and other physical properties, recover the cadmium as a valuable byproduct and secure a chemically pure zinc oxide.

I have discovered that the above results will be obtained when the following conditions are maintained during the procedure:

(1) It is important that the vapors over the zinc oxide circulate in order that sulfur impurities may be removed at relatively low temperatures. This circulation is just as important. for sulfur removal, as the composition of the atmosphere circulated over the zinc oxide.

For example. in Table I are given the results of reheating a zinc` oxide containing 0.177% sulfur.

TABLE I Reheatz'ng of crude zinc oxide Time of heating, minutes Per cent Tgnip.,

C. suliur Run No.

IN STAGNANT AIR The original crude AZO 55 It Will be observed that in stagnant air practically no removal of sulfur results after treatment for as long as 60 minutes and at temperatures as high as 700 C.l On the other hand when air is passed over the oxide at the rate of 50 to sages TABLE II Reheatz'na of crude zinc oxide in a reducing atmosphere Time of 'Per cent Temp. Per cent Run \Io. heating total L o C minutes cadmium sulfur 0.351 0.002 050 v 0.105 0. 157 650 0. 249 0.159 650 0,082 0.142 650 a0 0.078 0.165 650 70- 0 054 0 099 700 00 0. 039 0. 102 70o 120 0.05

A reducing 'atmosphere containing a mixture of Hz and CO was passed over the oxide at different temperatures. It will be noted that an oxide containing 0.351% cadmium and 0.392% sulfur may be reheated in this atmosphere for 60 minutes at 700\C. and the sulfur reduced to 0.102% and the cadmium content reduced to 0.039%.

l(3) By alternating the atmosphere from reducing to oxidizing or vice versa when the intervals are short, the reaction zone in the furnace is'of `sufficient length, and the movement of the oxide is properly coordinated, a further reduction-in cadmium and sulfur content occurs and. in addition, a color superior to any obtained heretofore by reheating is secured. Sulfur and cadmium removal may be accomplished without the oxidizing cycle but the color will not be enhanced by such treatment. However, by alternating from oxidizing to reducing or vice versa in the .furnace a superior color is obtained when treating ores containing oxides or other removable impurities. Desirable results may be obtained by operating two'reheating furnaces in tandem. the first under `reducing conditions and the `second under oxidizing conditions. The alternating is highly'desirable from the viewpoint of cadmium recovery. During the reduction step some of the cadmium is reduced yto cadmium sulfide.- During the oxidizing step this cadmium sulfide is oxidized to cadmium sulfate,- and in this form is more readily removed by reduction to metallic cadmium vapors.

. (4) The use of a clean reducing atmosphere has proven to be more 'effective in every'respect than the addition of lamp black or other solid carbonaceous material to the oxide. The purpose of adding carbonaceous material heretofore has been to 1produce mildly reducing conditions that assist in breaking the sulfates to sultes which latter are vaporized at lower temperatures, but when these small particles of carbon are burned they form minute local high temperature reduction zones or spots in the charge. The tempera? ture of these spots may momentarily go as high as. 11200 C.. to. 1400 C.; which is gwell aboyethe point where objectionable densification occurs. At lthis high temperature not only are the compounds ofl sulfur and cadmium reduced but some zinc also is reduced to the metallic state in the vapor phase. These fzinc vvapors Amigrate rapidly away from the reducing `zone to colder region's vin the charge where they are oxidized and 'recondensed on the surface "of other zinc oxide vparticles. This results in fa "growth -in particle 'size' of the oxide, or in other words. densification. I

have found that when 'a carefully controlled reducizig atmosphere is used such as an atmosphere having va high percentage of carbon monoxide and hydrogen butwholly lfree of any carbon particles in suspension, i. e.,1soot, the sulfur Iand cadmium compounds are readily reduced `and volatilized at 4temperatures below the point where an appreciable amount of zinc -oxide is reduced:- and 'consequently 'there "is 'no densiflcation -or growth 'in particle size 'of the zinc oxide.

While I prefer :for present lpurposes to avoid densifl'cation it is nevertheless vpointed out 'that such densificatio'n may vbe secured by extending the period of the oxidizing cycle with respect to the reducing cycle.

(5) The'products of combustion'of a'fuel, when properly controlled to eliminate smoke, provide the 'proper atmosphere in the furnace `and Vat the same time provide heat sufficient to raise the zinc oxide to the required temperature Such gases should have the necessary velocity in order fto effectively remove the impurities 'andassist in the 'refining lprocess Sandan atmosphere controlled as to velocity, composition and temperature may be secured for the purposes of my process in lboth direct-fired gas furnaces and muiiie type furnaces. r

The velocity of the gases will vary dependent upon the length and interior diameter of the furnace or kiln. I have found that in a rotary furnace 70 feet long and 5 feet internal diameter thevelo'city of the gases at the lower end of the rotary furnace where the oxide is discharged is approximately 200v feet 'per minute and at this point the temperature of the gases may reach a maximum of 950 C. At a point midway lbetween the discharge end of the furnace and the end where the oxide is introduced the gases will have a velocity of approximately v yfeet per minute, and at this point the temperature of the -gases is approximately 650 C. which is approximately the -minimum vtemperature at which chemical reactions take place. .Between this point and the end `of' the furnace `where the oxide is introduced iis a preheating zone 'for the oxide. At the upper end of Vthe rotary .furnace .where the oxide lis introduced the velocity of "the gases is approximately [50] 95 feet per lminute and the temperature of the .gases at this'point is `approximately 300 C.

It will be noted that, :in considering the velocity of a .gas the 'temperature must also be 'considered because, y.for example, [2000] 20,000 cu. ft. of gas at room temperature (say 25 C.) expandsto [51,000] approximate# ly 78,600 cu. ft. at 900 C.; and asnoted in the furnace `just indicated :the temperature range ofthe gases is between 1900" C. and' 300 C. Substantially the same weight of gas is passing through the end of the furnace where the temperature of the. gases is 300P C. as is passing through the 'hot end but the volume has been materially reduced by the cooling. Itisfor. this reason that the velocities range the furnace where the temperature of theV gases ls approximately 300 C.

At standard conditionsw" C'. and atmospheric pressure), the Same weight of gas would have a volume such that its average linear velocity through the furnace wuold be reduced to about 50 feet per minute or less. t

The zinc oxide dustthat passes outythro-ugh the discharge 2a of the bonnet 2 issmall in quantity and vlow in cadmium content. Most of the dust falls out -of the stream as the velocity drops, and mixes with the downwardly flowing oxide. l

l(6) I have found that none of the desira- 'ble physical properties are sacrificed with the removal of cadmium as an impurity from the zinc oxide; and, further by the removal of the sulfur anclcadmium I have been able to produce an American process zinc oxide equal in purity `and quality to the-French process zinc oxide for many purposes, for example, for use in quick-curing rubber compounds. Further the' present refiningv process is the firstl of which -f-have knowledge that simultaneously recovers the cadmium as Aa valuable by-product. In many American process zinc 'oxides treated by my process Vthe value of the cadmium recovered more than covers the total refining expense. f

The chemicalY composition of the cadmium and sulfur impurtiesr'which have not been removed are of extreme importance because of theirbehavior when the zinc oxide is subsequently used for certain purposes. For ex ample, small percentagesv of cadmium oxide or sulfate have a very detrimental eiect if 1- the zinc oxide is used in 'compounding rubber with thiuram accelerators. However, cadmium sulfide has no detrimental effect. Therefore I control the atmosp-here in the furnace in such a 'fashion that any residual cadmium will be there as cadmium sulfide.

Similarly the S04 ion is objectionable in certain rubber compounds but sulfurpresent as sulfide has no detrimental effect. Therefore, the ratio of reducing to oxidizing time is manipulated so as to leave no residual sulfur in the yform of sull fate `nor any residual cadmium in the form of oxide or` sulfate. Ifv it is desired to retain the sulfur compounds assulildes and the cadmium as sulfide the zine oxide is `cooled in a nonoxidizing atmosphere" after it is discharged from the kiln. If for anyreason it is desired to have the residual sulfurcompounds in the form of sulfate, it isf only necessary to allow the zinc oxide to cool-in the presence of air.

. In Tables III and IV are presented data illustrating the effect of non-oxiding cooling upon both the water soluble cadmium and the water soluble, sulfurcontents: Zinc oxides rei-med in the Wemple refinery have water soluble sulfurs running from 46 to 100% of the total sulfur content. On the other'hanchwater solublesulfurs in oxides refined by my improved process and cooled in a non-oxidizing atmosphere do no exceed of the total sulfur. content; In the case of cadmium my improved product cooled in a non-oxidizing atmosphere contains no water soluble cadmium andwhen cooled in an oxidizing atmosphere only about one-fifth of the total cadmium is present in aiwater soluble form. 0n the-otherhand oxides refined in the. Wemple furnace have between itwo-thirds -and three-l fourths of their cadmium content in a water soluble form.

TABLE III Sulfur l iiumber` *A lgguct Cooled Total `Soluble 0.002 0.00 020 Nail-oxidizing.

.085 .002 2.4 -I 'Do. .030 .002 6.2 D0. .0411v 0032 47. Sv D0. .135 .0032 2. 4 Do. Ll14 .002 1.8 Do. .035 .016 46.0 Oxldizing (Wemple Y refinery). .145 +100. 0 D0.

.120 .'112 foal; D0.

- TABLE IV Cadmium Number lgufl; Cooled Total Soluble 0. 211 0. 00 0. 0 Non-oxidizing.

.141 .0() 0.0 Do. l .050 .011 21.0 Oxldizlng. .057 .011 19.0 D0. .560 .361 64.5 Oxldizing (Wemple f refinery). v- 442 .269 61.0 D0.

.25 78.0 Do. l .25 13.5 no.

' It is to be understood that muille'type kilns' or furnaces provided with devices for controlling the atmosphere in the kiln with. respect to its composition, temperature and velocity maybe used in performing my process; and, as noted, satisfactory results may be obtained by operating two heating automatically-controlled furnaces arranged in tandem, the rst operating under controlled reducing conditions andthe second under controlled oxidizing conditions. However, in' carrying out my invention, I preferably make use --of a gas-fired rotary kiln of the type commonly used for calcining materials by direct conf tact with the gases of combustion. Such a kiln may be approximately 70 feet long lined with brick and having 5 feet inside diameter. Air seals should be disposed at both ends of the kiln to 'prevent accidental or casual admission ofair; Using this type of kiln the zinc oxide isfadmitted at the high end of the kiln and is conveyed by the rotary motion ofthe kiln to the low or discharge end where it is 'discharged through a. sealinto a suitable receptacle. The zinc oxide moves `countercurrent to the flow of th' combus tion gases which are admitted at the low or discharge end ofV the kiln. The oxide should be retained in the reactionzone' of the kiln for v15 to 30 minutes. 1 The ratio of air to fuel admitted tothe burners is automatically controlled. 'I prefer to use as an `oxidizing flame that 'flame whichA is obtained when natural gas is burned with not less than/10%' excess of air, and a reducing flame which is obtained when natural gas is burned vwith not less than 30% deficiency of air. Flames of similar composition can be` had by the con-v trolled combustion of otherfuels such as butano. producer or water gas. The change from an oxidizing condition -toa reducing condition inthe kilnv should not be haphazard', but ,the shiftshould .be made 'at regular intervals" andi for a. predeter mined period, of time. As, at present. practiced I prefer to have the burner operateunder oxidizing conditions for about minutes and then shift promptly to a reducing condition and operate under theseconditions, for aboutl 5' minutes. The temperature and time. o ff retention of the zinc oxide in the, reaction zone will determine whether one or more alter-nations is required forI complete refining'.4 Conditions sometimes require a, different. ratio of oxidizing to reducing time.` This may be done by an electrical timer that provides an adjustable ratio of reducing to oxidizingtime. The temperature of' the zinc oxide should not be higher than 950 C. just before it is discharged from the kiln. As mentioned above, this temperiature is higher (perhaps 200 C.) than may be possible in other types of furnaces or kilns without resulting in densiiication.

` I have shown in the accompanying drawing, forP illustrative purposes only, one means for performing the present zinc refining and cadmium recovering process which apparatusA is not claimed herein butv which will be the subject matter of; a separate application. and wherein Fig. 1 is a side elevation partly in section of my preferred apparatus; and

Figs; 2 and 3 are enlarged detail views of the valves for controlling the supply of combustion fuel to the combustion chamber showing the valves in oxidizing and reducing positions. respectively.

In said drawing I vis a brick-lined rotary kiln of conventional design at each end of' which are bonnets 2 and 3'. Air seals 4l and 5 are provided to prevent the entrance of casual air into the kiln while it is operating. The zine oxide is fed into the kiln by means of a screw 6. Anyy dust and/or cadmium fumes driven from the zinc oxide bythe reheating are drawn into a dust collecting unity 1J by means. oi a fan 8. A flue 9 having a.

butterfly valve 9a which is normally' closed is provided. In the event of failure of the dustcollecting system 1 and 8l the valve 9a is opened and the fumes pass out through the flue 9. The hot zinc` oxide is discharged from the furnace into awater-cooled conveyor I0, and the oxide is then discharged through an outlet Il into a receptacle I2 having an air-sealing connection at I 3 with the outlet I I.

The kiln is heated by means of a burner I4 which is associated with a combustion chamber I4a to prevent a smokey flame. The burner I4 is provided with mixers I5 and I6 which are adapted to mix the air and fuel. In the furnace illustrated natural gas is delivered to these mixers at low pressure by means of a pair of' gas supply regulators I'I and I8. A blower I9 delivers the air supply to the mixers through the pipes 2U and 2l to a pair ofv butterfly valves 2 2 and 23 the former controlling the reducing gas and the latter the oxidizing gas. These valves are automatically actuated by the solenoid 24. Said but-v terilyv valves are connectedv to the. solenoid 2.4 by connecting links 25, 26' and 2T so. that when 011.6 valve is open the other is closed.

The adjustments on the two mixers I5v and I6 switch is setfor this period, thev solenoid is. again energized and the flame is changed to a reducing one by closing the valve 23 and opening the valve 22. as shown in Fig.- 3. The Valves 22 and 23 are moved fromv opened to closed position as rapidly as possible during operation'l of the burner. Likewise when one mixer isv supplying gas the other mixer is closed.

Usually the time-controlled switch 28 is set, to provide 5 minutes of oxidizingliiame and 5 minutes o f reducing flame, but other ratios and other intervals may be provided as required.

A gas analysis device 2S is connected at 30 and samples are taken from time to time and analyzed. 'Iypical analyses of reducing andv oxidizng eases as taken, at thisl point are shown inV Table V-v TABLE` V lie-orl a reducing atmosphere, the following are typical analyses For an oxidizing atmosphere, the following is a typical example.

The reducing mixer-is adjusted to avoid a smokey flame.

A, steam jet is provided at 3| projecting through the wall of the combustion chamber I4a to supply steam to the burner, or a jet to supply an inert gas maybe. substituted therefor. Although the Volume and temperature of the combustion gases may be such as to provide the required temperature and a suhcient volume of gases. to provide volatilization of the impurities; from the z inc oxide charge, normally a greater volumev and an identical percentage of reducing gas without increasing the heat input of the furnace should be provided by adding steam orinert gas to the name. 'I headdition of steam or such inert gasy provides a means for obtaining a higher velocity of gases without increasing the temperature. The jet of steam or inert gasat 3| has the-additional func.- tion of modifying and controlling the temperature.l of the gases in the. furnace. The temperatureA of thegases atV the burner I4 in the combustion chamber` is approximately l500 C. and this temperature is reduced by thel action of the jet of steam or inert gas to about 9570? C. at the discharge end of the furnace as heretofore stated. As. indicated on4 Fig. l the reaction zone extends between4 a variable point A inthe furnace,r which is; determined by the rate of feed of the oxide and ther temperature of the gases, and thepoint of discharge B.4 The area between the point A Y 11 and the point C where the oxide is introduced constitutes a preheating zone for the oxide.

Further, oxidation of residual sulfldes of zinc or cadmium can be prevented during the cooling of the oxide by steam or inert gas admitted at 32 to the cooling conveyor l0.

The minimum refining temperature is about 650 C. Temperatures may be determined'by Ysuitably placed pyrometers such as the one located at 33 adjacent to the discharge end of the kiln. I have found that excellent results may .be obtained at '750 C. However, inasmuch as densication does not occur at temperatures as high as 950 C. in a controlled atmosphere for short periods, production may be facilitated by allowing the oxide to reach that temperature just before it is discharged from the heating zone of the furnace.

The following results. among others, will be secured with this process as heretofore described:

(l) Zinc Vsulfate and other zinc-sulfur compounds in the zinc oxide will be decomposed and sulfur therein volatilized and will pass out of the kiln in the gas stream.

(2) Certain metallic impurities of the zincoxide such as cadmium, arsenic, and antimony [and lead] compounds will be volatilized and will also pass out of the kiln in the gas stream.

(3) Any carbonaceous material present in the 'oxide will be oxidized during the oxidizing cycle.

(4) The cadmium values in the emitted gas stream will be recovered by the fume-collecting mechanism; and approximately 90% of the cadmium in the crude is being recovered. .This cadmium so recovered consists of a mixture of cadmium dust (metallic), cadmium oxide, and cadmium sulfate. y

(5) The color is improved by the removal of the carbonaceous material and metallic impurities by the oxidizing and reducing cycles, and

the product is free of densiiication or increase in particle size and hence its texture, oil absorpltion and other physical characteristics are not adversely affected by the refining action.

Thus there is produced a reheated product substantially free from carbonaceous material, sulfur compounds, cadmium, etc.; and the product is not only of exceptional chemical purity and whiteness, but the texture, oil absorption and other physical properties remain substantially unchanges due to the elimination of densification or'particle growth, and the cadmium is recovered as a valuable by-product.

What is claimed is:

[1. A process of refining pigment zinc oxide which comprises reheating theoxide in a clean reducing atmosphere free of soot and solid carbon and removing superficial impurities at a temperature below the decomposition temperature of the zinc oxide] [2. A process of rening pigment zinc oxide which consists in caleining the oxide by direct contact with the hot gases of combustion of the fuel which are controlled as to gaseous composition, temperature and velocity to remove supercial and admixed impurities under reducing conditions that prevent a substantial decomposition of the zinc oxide.]

[3. A process of refining pigment zinc oxide which consists in reheating the oxide in a stream of hot, clean, reducing gases moving at a Velocity between 50 and 200 feet per minute and at temperatures below the decomposition temperature of the zinc oxide to remove admixed impurities] 12 Y v 4. A process of refining zinc oxide which consists in heating crude pigment zinc oxide in a clean reducing atmosphere at a temperature of not less than 650 C. and not more than 950 C., said atmosphere passing over` the oxide at a velocity varying from 50 to 200 feet per minute.

5. A process for lrefining pigment zinc oxide comprising heating the oxide in a reducing atnicsphere at a temperature between 650 C. and 950 C., said atmosphere moving over the oxide at a velocity of at least 50 feet per minute, and then continuing the heating at the same temperature range in an oxidizing atmosphere for a similar period. v

6. The process of calcining pigment'zinc oxide which comprises heating the zinc'oxide to remove admixed impurities by an atmosphere movingover the Ioxide at a velocity within the range of 50 to 200 feet per minute which is alternated from oxidizing to reducing periodically and at temperatures [below the decomposition temperature of the zinc oxide] between 650 C. and 950 C.

['7. The process of calcining zinc oxide to purify the same by removingsuperciall or admixed impurities which comprises heating the zinc oxide in a llame which is periodically alternated from oxidizing to reducing and wherein the flame is free of solid carbon and the velocity of which is controlled and at temperatures below the decomposition temperature of the zinc oxide] 8. A process of refining pigment 'zinc oxide which consists lin passing the oxide through a furnace countercurrent to the fiow of heating gases produced by the combustion of gaseous fuel and moving over the oxide at la velocity within the range of 50 to 200v feet per minute and maintained free of soot and solid carbon and periodically alternating at short intervals from a reducing to an oxidizing composition and at temperatures [below the decomposition temperature of zinc oxide] between-650 C. and .950 C'.

9. An improvement in the refining of pigment zinc oxide which consists in'calcining the oxide at a temperature [of less than] between, 650 C. and 950 C. in direct contact'with the combustion gases moving over the oxide at a velocity of at least 50 feet per minute to remove super ficial or admixed impurities, such gases alter nating from oxidizing to reducing at short intervals and repeating the alternation until puriiied [at temperatures below the decomposition temperature of the zinc oxide] 10. The process of purifying zinc oxide which comprises reheating it at a temperature between 650 C. and 950C. in direct contact with a ame maintained free of soot and solid carbon to remove superficial or admixed impurities. alternating the composition of the flame at short intervals from an oxidizing condition to a reducing condition, moving the llame over the oxide of a velocity of not less than 50Y feet per minute, and maintaining thefoxide in such an atmosphere until completely refined.

11. A process for refining zinc oxide to remove superficial or admixed impurities comprising a1- ternately heating the oxide in a reducing atmosphere at a temperature between 650 C. and 950 C. for about 5 minutes and then continuing the heating at the same temperature range in an oxidizing atmosphere for about 5 minutes said atmospheres moving over the oxide at a velocity of at least 50 feet per minute.

12. A process for refining or purifying zinc oxide which comprises reheating the zinc oxide to a temperature between 650 C. and 950 C. in a clean reducing atmosphere moving over the xide at a. velocity of not less than 50 feet per minute to remove superficial or admixed impurities [at temperatures below the decomposition temperature of the zinc oxidel, removing the oxide from the heating zone, and cooling while maintaining a reducing atmosphere about the oxide.

[13. A continuous process for refining Zinc oxide to remove sulfur and cadmium impurities which comprises feeding zinc oxide into one end of an air-sealed furnace and into direct contact with a high velocity heating gas moving countercurrently to the ow of zinc oxide, maintaining the reaction zone of the furnace at a temperature of from 650 C. to 950 C., said gases also being maintained clean and free of soot and solid carbon, and periodically alternating the gases at short intervals of time from a reducing composition to an oxidizing composition] 14. A continuous process for refining zinc oxide to remove sulfur and cadmium impurities which comprises feeding zinc oxide into an airsealed rotary furnace having a reaction zone maintained at temperatures between 650 C. and 950 C. and into direct contact with heating gases, said gases [being supplied at a relatively high velocity] moving through the reaction zone at a velocity within the range of 50 to 200 feet per minute countercurrently to the flow of the zinc oxide, maintaining the gases clean and free of soot and solid carbon, periodically alternating the gases from a reducing composition to an oxidizing composition, and finally discharging the refined zinc oxide, from the other end of the furnace through a zone at about 950 C.

15. A continuous process for refining zinc oxide to remove sulfur and cadmium impurities v which comprises feeding zinc oxide into one end of an air-sealed furnace and into direct contact with a [high velocity] heating gas moving countercurrently to the flow of zinc oxide through the reaction zone at a velocity within the range of 50 to 200 feet per minute, maintaining thev short intervals of time from a reducing composition having a 40% deficiency of air to an oxidizing composition having a 40% excess of air and finally discharging the rened zinc oxide from the opposite end of the furnace through a zone at about 950 C. into a non-oxidizing atmospherefl.

16. A continuous process for rening zinc oxide to remove sulfur and cadmium impurities Which comprises feeding zinc oxide into an airsealed rotary furnace having a reaction zone maintained at temperatures between 650 C. and 950 C. and into direct contact with heating gases, said gases being prepared by diluting a high temperature flame with steam or other inert gas, passing said gases countercurrently to the flow of the zinc oxide through the reaction lzone at a. velocity within the range of 50 to 200 feet per minute, maintaining the gases clean and free of soot and solid carbon, alternating the gases from a reducing composition to an oxidizing composition, and finally discharging the zinc oxide from the furnace.

[17. A process for recovering the cadmium from pigment zinc oxide which consists in heating the oxide in a clean and highly reducing atmosphere and a temperature not lower than 650 C. and not higher than 950 C., and collecting the emitted vapors to recover the cadmium] 18. A process for recovering the cadmium in zinc oxide which consists of calcining the oxide in a substantial reducing atmosphere at a temperature between [900 C.] 950 C. and 650 C., said atmosphere passing over the oxide at a velocity between 50 and 200 feet per minute, and collecting the emitted vapors to recover the cadmium.

JOHN HENRY CALBECK.

REFERENCES CITED The following references are of record in the file of this patent or the original patent.

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